Scalable, real-time messaging system

ABSTRACT

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for balancing loads in a publish-subscribe system. An example method includes: selecting a first hosting node from a plurality of hosting nodes based, at least in part, on a load data selected from at least one of node-specific data representing loads on the plurality of hosting nodes and channel-specific data representing a load associated with a channel; sending a request to the first hosting node to temporarily host a portion of a channel; temporarily hosting the channel portion by the first hosting node by temporarily storing one or more messages published to the channel, and temporarily providing, to a plurality of subscribers to the channel, access to the one of more messages; receiving a request to access the channel portion; and granting permission to access the channel portion.

BACKGROUND

This specification relates to a data communication system and, inparticular, a system that implements real-time, scalablepublish-subscribe messaging.

The publish-subscribe pattern (or “PubSub”) is a data communicationmessaging arrangement implemented by software systems where so-calledpublishers publish messages to topics and so-called subscribers receivethe messages pertaining to particular topics to which they aresubscribed. There can be one or more publishers per topic and publishersgenerally have no knowledge of what subscribers, if any, will receivethe published messages. Some PubSub systems do not cache messages orhave small caches meaning that subscribers may not receive messages thatwere published before the time of subscription to a particular topic.PubSub systems can be susceptible to performance instability duringsurges of message publications or as the number of subscribers to aparticular topic increases.

SUMMARY

In general, one aspect of the subject matter described in thisspecification can be embodied in a computer-implemented load-balancingmethod for a publish-subscribe system. The method includes: selecting afirst hosting node from a plurality of hosting nodes based, at least inpart, on load data selected from at least one of node-specific datarepresenting loads on the plurality of hosting nodes andchannel-specific data representing a load associated with a channel;sending a request to the first hosting node to temporarily host aportion of the channel; temporarily hosting the channel portion by thefirst hosting node by temporarily storing one or more messages publishedto the channel, and temporarily providing, to a plurality of subscribersto the channel, access to the one or more messages; receiving a requestto access the channel portion; and granting permission to access thechannel portion.

In certain examples, the node-specific data includes one or more loadmetrics, which can be or include, for example: a number of channelportions being temporarily hosted by the respective hosting nodes, anumber of interface nodes having permission to access the respectivehosting nodes, a data reception rate of the respective hosting nodes, adata transmission rate of the respective hosting nodes, a storageutilization of the respective hosting nodes, and/or a processing rate ofthe respective hosting nodes. The method can include receiving at leasta portion of the node-specific data from the plurality of hosting nodes.Alternatively or additionally, the method can include determining atleast a portion of the node-specific data based, at least in part, onreceived requests to access channel portions and on permissions grantedto access channel portions.

In some examples, the channel-specific data includes one or more loadmetrics, which can be or include, for example: a number of subscribersto the channel, a number of publishers to the channel, a rate at whichmessages are published to the channel, a rate at which messages are readfrom the channel, a number of interface nodes having permission toaccess the channel, and/or a channel portion size for the channel. Themethod can include receiving at least a portion of the channel-specificdata from at least one of a hosting node and an interface node.Selecting the first hosting node from the plurality of hosting nodesbased, at least in part, on the load data can include: determining,based at least in part on the node-specific data, that a load on thefirst hosting node is lowest among respective loads on the hostingnodes; and selecting the first hosting node based, at least in part, onthe determination. In some instances, selecting the first hosting nodefrom the plurality of hosting nodes based, at least in part, on the loaddata includes: determining, based at least in part on the node-specificdata, that a load on the first hosting node is below a threshold loadlevel; and selecting the first hosting node based, at least in part, onthe determination.

In various implementations, selecting the first hosting node from theplurality of hosting nodes based, at least in part, on the load dataincludes: determining, based at least in part on a portion of thenode-specific data corresponding to the first hosting node and on aportion of the channel-specific data corresponding to the channel, anexpected load on the first hosting node that would result from the firsthosting node hosting the portion of the channel; determining that theexpected load on the first hosting node is below a threshold load level;and selecting the first hosting node based, at least in part, on thedetermination that the expected load on the first hosting node is belowthe threshold load level. The channel portion can include a firstportion of the channel, the channel can include a second portion, andselecting the first hosting node from the plurality of hosting nodesbased, at least in part, on the load data can include: determining thatthe first hosting node hosts the second channel portion; determiningthat a load on the first hosting node is below a threshold load level;and selecting the first hosting node based, at least in part, on thedeterminations that the first hosting node hosts the second channelportion and that the load on the first hosting node is below thethreshold load level.

In certain examples, the channel portion includes a first portion of thechannel, the channel further includes a second portion, and selectingthe first hosting node from the plurality of hosting nodes based, atleast in part, on the load data includes: determining that a secondhosting node hosts the second channel portion; determining that a loadon the second hosting node is above a threshold load level; determiningthat a load on the first hosting node is below the threshold load level;and selecting the first hosting node based, at least in part, on thedeterminations that the load on the second hosting node is above thethreshold load level and that the load on the first hosting node isbelow the threshold load level. In one example, selecting the firsthosting node from the plurality of hosting nodes based, at least inpart, on the load data includes: determining, based at least in part ona portion of the channel-specific data, an expected load associated withhosting the channel portion; determining, based at least in part on thenode-specific data and on the expected load associated with hosting thechannel portion, that hosting the channel portion on the first hostingnode would reduce inequality of load distribution among the hostingnodes; and selecting the first hosting node based, at least in part, onthe determination that hosting the channel portion on the first hostingnode would reduce inequality of load distribution among the hostingnodes.

In another aspect, the subject matter of this specification relates to apublish-subscribe system having a plurality of hosting nodes, aninterface node, and a channel manager node. The system is operable toperform operations including: selecting a first hosting node from theplurality of hosting nodes based, at least in part, on a load dataselected from at least one of node-specific data representing loads onthe plurality of hosting nodes and channel-specific data representing aload associated with a channel; sending a request to the first hostingnode to temporarily host a portion of the channel, wherein the firsthosting node temporarily hosts the channel portion by temporarilystoring one or more messages published to the channel, and temporarilyproviding, to a plurality of subscribers to the channel, access to theone or more messages; receiving, from the interface node, a request toaccess the channel portion; and granting, to the interface node,permission to access the channel portion.

In various instances, the operation of selecting the first hosting nodefrom the plurality of hosting nodes based, at least in part, on the loaddata includes: determining, based at least in part on the node-specificdata, that a load on the first hosting node is lowest among respectiveloads on the hosting nodes; and selecting the first hosting node based,at least in part, on the determination. The operation of selecting thefirst hosting node from the plurality of hosting nodes based, at leastin part, on the load data can include: determining, based at least inpart on the node-specific data, that a load on the first hosting node isbelow a threshold load level; and selecting the first hosting nodebased, at least in part, on the determination. In some examples, theoperation of selecting the first hosting node from the plurality ofhosting nodes based, at least in part, on the load data includes:determining, based at least in part on a portion of the node-specificdata corresponding to the first hosting node and on a portion of thechannel-specific data corresponding to the channel, an expected load onthe first hosting node that would result from the first hosting nodehosting the portion of the channel; determining that the expected loadon the first hosting node is below a threshold load level; and selectingthe first hosting node based, at least in part, on the determinationthat the expected load on the first hosting node is below the thresholdload level.

In certain implementations, the channel portion includes a first portionof the channel, the channel further includes a second portion, and theoperation of selecting the first hosting node from the plurality ofhosting nodes based, at least in part, on the load data includes:determining that the first hosting node hosts the second channelportion; determining that a load on the first hosting node is below athreshold load level; and selecting the first hosting node based, atleast in part, on the determinations that the first hosting node hoststhe second channel portion and that the load on the first hosting nodeis below the threshold load level. In some instances, the channelportion includes a first portion of the channel, the channel furtherincludes a second portion, and the operation of selecting the firsthosting node from the plurality of hosting nodes based, at least inpart, on the load data includes: determining that a second hosting nodehosts the second channel portion; determining that a load on the secondhosting node is above a threshold load level; determining that a load onthe first hosting node is below the threshold load level; and selectingthe first hosting node based, at least in part, on the determinationsthat the load on the second hosting node is above the threshold loadlevel and that the load on the first hosting node is below the thresholdload level. In one example, the operation of selecting the first hostingnode from the plurality of hosting nodes based, at least in part, on theload data includes: determining, based at least in part on a portion ofthe channel-specific data, an expected load associated with hosting thechannel portion; determining, based at least in part on thenode-specific data and on the expected load associated with hosting thechannel portion, that hosting the channel portion on the first hostingnode would reduce inequality of load distribution among the hostingnodes; and selecting the first hosting node based, at least in part, onthe determination that hosting the channel portion on the first hostingnode would reduce inequality of load distribution among the hostingnodes.

In another aspect, the subject matter of this specification relates toan article that includes a non-transitory machine-readable medium havinginstructions stored thereon that when executed by one or more computerscauses the computers to perform operations including: selecting a firsthosting node from a plurality of hosting nodes based, at least in part,on a load data selected from at least one of node-specific datarepresenting loads on the plurality of hosting nodes andchannel-specific data representing a load associated with a channel;sending, to the first hosting node, a request to temporarily host aportion of the channel; temporarily hosting the channel portion by thefirst hosting node by temporarily storing one or more messages publishedto the channel, and temporarily providing, to a plurality of subscribersto the channel, access to the one or more messages; receiving a requestto access the channel portion; and granting permission to access thechannel portion.

Elements of embodiments or examples described with respect to a givenaspect of the invention can be used in various embodiments or examplesof another aspect of the invention. For example, it is contemplated thatfeatures of dependent claims depending from one independent claim can beused in apparatus, systems, and/or methods of any of the otherindependent claims.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an example system that supports the Pub Subcommunication pattern.

FIG. 1B illustrates functional layers of software on an example clientdevice.

FIG. 2 is a diagram of an example messaging system.

FIG. 3A is a data flow diagram of an example method for writing data toa streamlet.

FIG. 3B is a data flow diagram of an example method for reading datafrom a streamlet.

FIG. 4A is a data flow diagram of an example method for publishingmessages to a channel of a messaging system.

FIG. 4B is a data flow diagram of an example method for subscribing to achannel of a messaging system.

FIG. 4C is an example data structure for storing messages of a channelof a messaging system.

FIG. 5 is a flowchart of an example method for storing messages of amessaging system.

DETAILED DESCRIPTION

FIG. 1A illustrates an example system 100 that supports the Pub Subcommunication pattern. Publisher clients (e.g., Publisher 1) can publishmessages to named channels (e.g., “Channel 1”) by way of the system 100.A message can comprise any type of information including one or more ofthe following: text, image content, sound content, multimedia content,video content, binary data, and so on. Other types of message data arepossible. Subscriber clients (e.g., Subscriber 2) can subscribe to anamed channel using the system 100 and start receiving messages whichoccur after the subscription request or from a given position (e.g., amessage number or time offset). A client can be both a publisher and asubscriber.

Depending on the configuration, a PubSub system can be categorized asfollows:

-   -   One to One (1:1). In this configuration there is one publisher        and one subscriber per channel. A typical use case is private        messaging.    -   One to Many (1:N). In this configuration there is one publisher        and multiple subscribers per channel. Typical use cases are        broadcasting messages (e.g., stock prices).    -   Many to Many (M:N). In this configuration there are many        publishers publishing to a single channel. The messages are then        delivered to multiple subscribers. Typical use cases are map        applications.

There is no separate operation needed to create a named channel. Achannel is created implicitly when the channel is subscribed to or whena message is published to the channel. In some implementations, channelnames can be qualified by a name space. A name space comprises one ormore channel names. Different name spaces can have the same channelnames without causing ambiguity. The name space name can be a prefix ofa channel name where the name space and channel name are separated by adot or other suitable separator. In some implementations, name spacescan be used when specifying channel authorization settings. Forinstance, the messaging system 100 may have app1.foo andapp1.system.notifications channels where “app1” is the name of the namespace. The system can allow clients to subscribe and publish to theapp1.foo channel. However, clients can only subscribe to, but notpublish to the app1.system.notifications channel.

FIG. 1B illustrates functional layers of software on an example clientdevice. A client device (e.g., client 102) is a data processingapparatus such as, for example, a personal computer, a laptop computer,a tablet computer, a smart phone, a smart watch, or a server computer.Other types of client devices are possible. The application layer 104comprises the end-user application(s) that will integrate with the PubSub system 100. The messaging layer 106 is a programmatic interface forthe application layer 104 to utilize services of the system 100 such aschannel subscription, message publication, message retrieval, userauthentication, and user authorization. In some implementations, themessages passed to and from the messaging layer 106 are encoded asJavaScript Object Notation (JSON) objects. Other message encodingschemes are possible.

The operating system 108 layer comprises the operating system softwareon the client 102. In various implementations, messages can be sent andreceived to/from the system 100 using persistent or non-persistentconnections. Persistent connections can be created using, for example,network sockets. A transport protocol such as TCP/IP layer 112implements the Transport Control Protocol/Internet Protocolcommunication with the system 100 that can be used by the messaginglayer 106 to send messages over connections to the system 100. Othercommunication protocols are possible including, for example, UserDatagram Protocol (UDP). In further implementations, an optionalTransport Layer Security (TLS) layer 110 can be employed to ensure theconfidentiality of the messages.

FIG. 2 is a diagram of an example messaging system 100. The system 100provides functionality for implementing PubSub communication patterns.The system comprises software components and storage that can bedeployed at one or more data centers 122 in one or more geographiclocations, for example. The system comprises MX nodes (e.g., MX nodes ormultiplexer nodes 202, 204 and 206), Q nodes (e.g., Q nodes or queuenodes 208, 210 and 212), one or more channel manager nodes (e.g.,channel managers 214, 215), and optionally one or more C nodes (e.g., Cnodes or cache nodes 220 and 222). Each node can execute in a virtualmachine or on a physical machine (e.g., a data processing apparatus).Each MX node serves as a termination point for one or more publisherand/or subscriber connections through the external network 216. Theinternal communication among MX nodes, Q nodes, C nodes, and the channelmanager, is conducted over an internal network 218, for example. By wayof illustration, MX node 204 can be the terminus of a subscriberconnection from client 102. Each Q node buffers channel data forconsumption by the MX nodes. An ordered sequence of messages publishedto a channel is a logical channel stream. For example, if three clientspublish messages to a given channel, the combined messages published bythe clients comprise a channel stream. Messages can be ordered in achannel stream, for example, by time of publication by the client, bytime of receipt by an MX node, or by time of receipt by a Q node. Otherways for ordering messages in a channel stream are possible. In the casewhere more than one message would be assigned to the same position inthe order one of the messages can be chosen (e.g., randomly) to have alater sequence in the order. Each channel manager node is responsiblefor managing Q node load by splitting channel streams into so-calledstreamlets (also referred to herein as “channel portions”). Streamletsare discussed further below. The optional C nodes provide caching andload removal from the Q nodes. Q nodes may also be referred to herein as“hosting nodes.” MX nodes may also be referred to herein as “interfacenodes.”

In the example messaging system 100, one or more client devices(publishers and/or subscribers) establish respective persistentconnections (e.g., TCP connections) to an MX node (e.g., MX node 204).The MX node serves as a termination point for these connections. Forinstance, external messages (e.g., between respective client devices andthe MX node) carried by these connections can be encoded based on anexternal protocol (e.g., JSON). The MX node terminates the externalprotocol and translates the external messages to internal communication,and vice versa. The MX nodes publish and subscribe to streamlets onbehalf of clients. In this way, an MX node can multiplex and mergerequests of client devices subscribing for or publishing to the samechannel, thus representing multiple client devices as one, instead ofone by one.

In the example messaging system 100, a Q node (e.g., Q node 208) canstore one or more streamlets of one or more channel streams. A streamletis a data buffer for a portion of a channel stream. A streamlet willclose to writing when its storage is full. A streamlet will close toreading and writing and be de-allocated when its time-to-live (TTL) hasexpired. By way of illustration, a streamlet can have a maximum size of1 MB and a TTL of three minutes. Different channels can have streamletslimited by different sizes and/or by different TTLs. For instance,streamlets in one channel can exist for up to three minutes, whilestreamlets in another channel can exist for up to 10 minutes. In variousimplementations, a streamlet corresponds to a computing process runningon a Q node. The computing process can be terminated after thestreamlet's TTL has expired, thus freeing up computing resources (forthe streamlet) back to the Q node, for example.

When receiving a publish request from a client device, an MX node (e.g.,MX node 204) makes a request to a channel manager (e.g., channel manager214) to grant access to a streamlet to write the message beingpublished. Note, however, that if the MX node has already been grantedwrite access to a streamlet for the channel (and the channel has notbeen closed to writing), the MX node can write the message to thatstreamlet without having to request a grant to access the streamlet.Once a message is written to a streamlet for a channel, the message canbe read by MX nodes and provided to subscribers of that channel.

Similarly, when receiving a channel subscription request from a clientdevice, an MX node makes a request to a channel manager to grant accessto a streamlet for the channel from which messages are read. If the MXnode has already been granted read access to a streamlet for the channel(and the channel's TTL has not been closed to reading) the MX node canread messages from the streamlet without having to request a grant toaccess the streamlet. The read messages can then be forwarded to clientdevices that have subscribed to the channel. In various implementations,messages read from streamlets are cached by MX nodes so that MX nodescan reduce the number of times needed to read from the streamlets.

By way of illustration, an MX node can request a grant from the channelmanager that allows the MX node to store a block of data into astreamlet on a particular Q node that stores streamlets of theparticular channel. Example streamlet grant request and grant datastructures are as follows:

StreamletGrantRequest = {   ″channel″: string( )   ″mode″: ″read″ |″write″   “position”: 0 } StreamletGrantResponse = {   ″streamlet-id″:″abcdef82734987″,   ″limit-size″: 2000000, # 2 megabytes max  ″limit-msgs″: 5000, # 5 thousand messages max   ″limit-life″: 4000, #the grant is valid for 4 seconds   “q-node″: string( )   “position”: 0 }

The StreamletGrantRequest data structure stores the name of the streamchannel and a mode indicating whether the MX node intends on readingfrom or writing to the streamlet. The MX node sends theStreamletGrantRequest to a channel manager node. The channel managernode, in response, sends the MX node a StreamletGrantResponse datastructure. The StreamletGrantResponse contains an identifier of thestreamlet (streamlet-id), the maximum size of the streamlet(limit-size), the maximum number of messages that the streamlet canstore (limit-msgs), the TTL (limit-life), and an identifier of a Q node(q-node) on which the streamlet resides. The StreamletGrantRequest andStreamletGrantResponse can also have a position field that points to aposition in a streamlet (or a position in a channel) for reading fromthe streamlet.

A grant becomes invalid once the streamlet has closed. For example, astreamlet is closed to reading and writing once the streamlet's TTL hasexpired and a streamlet is closed to writing when the streamlet'sstorage is full. When a grant becomes invalid, the MX node can request anew grant from the channel manager to read from or write to a streamlet.The new grant will reference a different streamlet and will refer to thesame or a different Q node depending on where the new streamlet resides.

FIG. 3A is a data flow diagram of an example method for writing data toa streamlet in various embodiments. In FIG. 3A, when an MX node (e.g.,MX node 202) request to write to a streamlet is granted by a channelmanager (e.g., channel manager 214), as described before, the MX nodeestablishes a Transmission Control Protocol (TCP) connection with the Qnode (e.g., Q node 208) identified in the grant response received fromthe channel manager (302). A streamlet can be written concurrently bymultiple write grants (e.g., for messages published by multiplepublisher clients). Other types of connection protocols between the MXnode and the Q node are possible.

The MX node then sends a prepare-publish message with an identifier of astreamlet that the MX node wants to write to the Q node (304). Thestreamlet identifier and Q node identifier can be provided by thechannel manager in the write grant as described earlier. The Q nodehands over the message to a handler process 301 (e.g., a computingprocess running on the Q node) for the identified streamlet (306). Thehandler process can send to the MX node an acknowledgement (308). Afterreceiving the acknowledgement, the MX node starts writing (publishing)messages (e.g., 310, 312, 314, and 318) to the handler process, which inturns stores the received data in the identified streamlet. The handlerprocess can also send acknowledgements (316, 320) to the MX node for thereceived data. In some implementations, acknowledgements can bepiggy-backed or cumulative. For instance, the handler process can sendto the MX node an acknowledgement for every predetermined amount of datareceived (e.g., for every 100 messages received) or for everypredetermined time period (e.g., for every one millisecond). Otheracknowledgement scheduling algorithms, such as Nagle's algorithm, can beused.

If the streamlet can no longer accept published data (e.g., when thestreamlet is full), the handler process sends a Negative-Acknowledgement(NAK) message (330) indicating a problem, following by an EOF(end-of-file) message (332). In this way, the handler process closes theassociation with the MX node for the publish grant. The MX node can thenrequest a write grant for another streamlet from a channel manager ifthe MX node has additional messages to store.

FIG. 3B is a data flow diagram of an example method for reading datafrom a streamlet in various embodiments. In FIG. 3B, an MX node (e.g.,MX node 204) sends to a channel manager (e.g., channel manager 214) arequest for reading a particular channel starting from a particularmessage or time offset in the channel. The channel manager returns tothe MX node a read grant including an identifier of a streamletcontaining the particular message, a position in the streamletcorresponding to the particular message, and an identifier of a Q node(e.g., Q node 208) containing the particular streamlet. The MX node thenestablishes a TCP connection with the Q node (352). Other types ofconnection protocols between the MX node and the Q node are possible.

The MX node then sends to the Q node a subscribe message (354) with theidentifier of the streamlet (in the Q node) and the position in thestreamlet from which the MX node wants to read (356). The Q node handsover the subscribe message to a handler process 351 for the streamlet(356). The handler process can send to the MX node an acknowledgement(358). The handler process then sends messages (360, 364, 366), startingat the position in the streamlet, to the MX node. In someimplementations, the handler process can send all of the messages in thestreamlet to the MX node. After sending the last message in a particularstreamlet, the handler process can send a notification of the lastmessage to the MX node. The MX node can send to the channel manageranother request for another streamlet containing a next message in theparticular channel.

If the particular streamlet is closed (e.g., after its TTL has expired),the handler process can send an unsubscribe message (390), followed byan EOF message (392), to close the association with the MX node for theread grant. The MX node can close the association with the handlerprocess when the MX node moves to another streamlet for messages in theparticular channel (e.g., as instructed by the channel manager). The MXnode can also close the association with the handler process if the MXnode receives an unsubscribe message from a corresponding client device.

In various implementations, a streamlet can be written into and readfrom at the same time instance. For instance, there can be a valid readgrant and a valid write grant at the same time instance. In variousimplementations, a streamlet can be read concurrently by multiple readgrants (e.g., for channels subscribed to by multiple publisher clients).The handler process of the streamlet can order messages from concurrentwrite grants based on, for example, time-of-arrival, and store themessages based on the order. In this way, messages published to achannel from multiple publisher clients can be serialized and stored ina streamlet of the channel.

In the messaging system 100, one or more C nodes (e.g., C node 220) canoffload data transfers from one or more Q nodes. For instance, if thereare many MX nodes requesting streamlets from Q nodes for a particularchannel, the streamlets can be offloaded and cached in one or more Cnodes. The MX nodes (e.g., as instructed by read grants from a channelmanager) can read the streamlets from the C nodes instead.

As described above, messages for a channel in the messaging system 100are ordered in a channel stream. A channel manager (e.g., channelmanager 214) splits the channel stream into fixed-sized streamlets thateach reside on a respective Q node. In this way, storing a channelstream can be shared among many Q nodes; each Q node stores a portion(one or more streamlets) of the channel stream. More particularly, astreamlet can be stored in, for example, registers and/or dynamic memoryelements associated with a computing process on a Q node, thus avoidingthe need to access persistent, slower storage devices such as harddisks. This results in faster message access. The channel manager canalso balance load among Q nodes in the messaging system 100 bymonitoring respective workloads of the Q nodes and allocating streamletsin a way that avoids overloading any one Q node.

In various implementations, a channel manager maintains a listidentifying each active streamlet, the respective Q node on which thestreamlet resides, an identification of the position of the firstmessage in the streamlet, and whether the streamlet is closed forwriting. In some implementations, Q nodes notify the channel manager andany MX nodes that are publishing to a streamlet that the streamlet isclosed due to being full or when the streamlet's TTL has expired. When astreamlet is closed, the streamlet remains on the channel manager's listof active streamlets until the streamlet's TTL has expired so that MXnodes can continue to retrieve messages from the streamlet.

When an MX node requests a write grant for a given channel and there isnot a streamlet for the channel that can be written to, the channelmanager allocates a new streamlet on one of the Q nodes and returns theidentity of the streamlet and the Q node in the StreamletGrantResponse.Otherwise, the channel manager returns the identity of the currentlyopen for writing streamlet and corresponding Q node in theStreamletGrantResponse. MX nodes can publish messages to the streamletuntil the streamlet is full or the streamlet's TTL has expired, afterwhich a new streamlet can be allocated by the channel manager.

When an MX node requests a read grant for a given channel and there isnot a streamlet for the channel that can be read from, the channelmanager allocates a new streamlet on one of the Q nodes and returns theidentity of the streamlet and the Q node in the StreamletGrantResponse.Otherwise, the channel manager returns the identity of the streamlet andQ node that contains the position from which the MX node wishes to read.The Q node can then begin sending messages to the MX node from thestreamlet beginning at the specified position until there are no moremessages in the streamlet to send. When a new message is published to astreamlet, MX nodes that have subscribed to that streamlet will receivethe new message. If a streamlet's TTL has expired, the handler process351 sends an EOF message (392) to any MX nodes that are subscribed tothe streamlet.

As described earlier in reference to FIG. 2, the messaging system 100can include multiple channel managers (e.g., channel managers 214, 215).Multiple channel managers provide resiliency and prevent single point offailure. For instance, one channel manager can replicate lists ofstreamlets and current grants it maintains to another “slave” channelmanager. As for another example, multiple channel managers cancoordinate operations between them using distributed consensusprotocols, such as, for example, Paxos or Raft protocols.

FIG. 4A is a data flow diagram of an example method for publishingmessages to a channel of a messaging system. In FIG. 4A, publishers(e.g., publisher clients 402, 404, 406) publish messages to themessaging system 100 described earlier in reference to FIG. 2. Forinstance, publishers 402 respectively establish connections 411 and sendpublish requests to the MX node 202. Publishers 404 respectivelyestablish connections 413 and send publish requests to the MX node 206.Publishers 406 respectively establish connections 415 and send publishrequests to the MX node 204. Here, the MX nodes can communicate (417)with a channel manager (e.g., channel manager 214) and one or more Qnodes (e.g., Q nodes 212 and 208) in the messaging system 100 via theinternal network 218.

By way of illustration, each publish request (e.g., in JSON key/valuepairs) from a publisher to an MX node includes a channel name and amessage. The MX node (e.g., MX node 202) can assign the message in thepublish request to a distinct channel in the messaging system 100 basedon the channel name (e.g., “foo”) of the publish request. The MX nodecan confirm the assigned channel with the channel manager 214. If thechannel (specified in the subscribe request) does not yet exist in themessaging system 100, the channel manager can create and maintain a newchannel in the messaging system 100. For instance, the channel managercan maintain a new channel by maintaining a list identifying each activestreamlet of the channel's stream, the respective Q node on which thestreamlet resides, and identification of the positions of the first andlast messages in the streamlet as described earlier.

For messages of a particular channel, the MX node can store the messagesin one or more buffers or streamlets in the messaging system 100. Forinstance, the MX node 202 receives from the publishers 402 requests topublish messages M11, M12, M13, and M14 to a channel foo. The MX node206 receives from the publishers 404 requests to publish messages M78and M79 to the channel foo. The MX node 204 receives from the publishers406 requests to publish messages M26, M27, M28, M29, M30, and M31 to thechannel foo.

The MX nodes can identify one or more streamlets for storing messagesfor the channel foo. As described earlier, each MX node can request awrite grant from the channel manager 214 that allows the MX node tostore the messages in a streamlet of the channel foo. For instance, theMX node 202 receives a grant from the channel manager 214 to writemessages M11, M12, M13, and M14 to a streamlet 4101 on the Q node 212.The MX node 206 receives a grant from the channel manager 214 to writemessages M78 and M79 to the streamlet 4101. Here, the streamlet 4101 isthe last one (at the moment) of a sequence of streamlets of the channelstream 430 storing messages of the channel foo. The streamlet 4101 hasmessages (421) of the channel foo that were previously stored in thestreamlet 4101, but is still open, i.e., the streamlet 4101 still hasspace for storing more messages and the streamlet's TTL has not expired.

The MX node 202 can arrange the messages for the channel foo based onthe respective time that each message was received by the MX node 202,e.g., M11, M13, M14, M12 (422), and store the received messages asarranged in the streamlet 4101. That is, the MX node 202 receives M11first, followed by M13, M14, and M12. Similarly, the MX node 206 canarrange the messages for the channel foo based on their respective timethat each message was received by the MX node 206, e.g., M78, M79 (423),and store the received messages as arranged in the streamlet 4101. Otherarrangements or ordering of the messages for the channel are possible.

The MX node 202 (or MX node 206) can store the received messages usingthe method for writing data to a streamlet described earlier inreference to FIG. 3A, for example. In various implementations, the MXnode 202 (or MX node 206) can buffer (e.g., in a local data buffer) thereceived messages for the channel foo and store the received messages ina streamlet for the channel foo (e.g., streamlet 4101) when the bufferedmessages reach a predetermined number or size (e.g., 100 messages) orwhen a predetermined time (e.g., 50 milliseconds) has elapsed. Forinstance, the MX node 202 can store in the streamlet 100 messages at atime or in every 50 milliseconds. Other acknowledgement schedulingalgorithms, such as Nagle's algorithm, can be used.

In various implementations, the Q node 212 (e.g., a handler process)stores the messages of the channel foo in the streamlet 4101 in theorder as arranged by the MX node 202 and MX node 206. The Q node 212stores the messages of the channel foo in the streamlet 4101 in theorder the Q node 212 receives the messages. For instance, assume thatthe Q node 212 receives messages M78 (from the MX node 206) first,followed by messages M11 and M13 (from the MX node 202), M79 (from theMX node 206), and M14 and M12 (from the MX node 202). The Q node 212stores in the streamlet 4101 the messages in the order as received,e.g., M78, M11, M13, M79, M14, and M12, immediately after the messages421 that are already stored in the streamlet 4101. In this way, messagespublished to the channel foo from multiple publishers (e.g., 402, 404)can be serialized in a particular order and stored in the streamlet 4101of the channel foo. Different subscribers that subscribe to the channelfoo will receive messages of the channel foo in the same particularorder, as will be described in more detail in reference to FIG. 4B.

In the example of FIG. 4A, at a time instance after the message M12 wasstored in the streamlet 4101, the MX node 204 requests a grant from thechannel manager 214 to write to the channel foo. The channel manager 214provides the MX node 204 a grant to write messages to the streamlet4101, as the streamlet 4101 is still open for writing. The MX node 204arranges the messages for the channel foo based on the respective timethat each message was received by the MX node 204, e.g., M26, M27, M31,M29, M30, M28 (424), and stores the messages as arranged for the channelfoo.

By way of illustration, assume that the message M26 is stored to thelast available position of the streamlet 4101. As the streamlet 4101 isnow full, the Q node 212 sends to the MX node 204 a NAK message,following by an EOF message, to close the association with the MX node204 for the write grant, as described earlier in reference to FIG. 3A.The MX node 204 then requests another write grant from the channelmanager 214 for additional messages (e.g., M27, M31, and so on) for thechannel foo.

The channel manager 214 can monitor available Q nodes in the messagingsystem 100 for their respective workloads (e.g., how many streamlets areresiding in each Q node). The channel manager 214 can allocate astreamlet for the write request from the MX node 204 such thatoverloading (e.g., too many streamlets or too many read or write grants)can be avoided for any given Q node. For instance, the channel manager214 can identify a least loaded Q node in the messaging system 100 andallocate a new streamlet on the least loaded Q node for write requestsfrom the MX node 204. In the example of FIG. 4A, the channel manager 214allocates a new streamlet 4102 on the Q node 208 and provides a writegrant to the MX node 204 to write messages for the channel foo to thestreamlet 4102. As shown in FIG. 4A, the Q node stores in the streamlet4102 the messages from the MX node 204 in an order as arranged by the MXnode 204: M27, M31, M29, M30, and M28 (assuming that there is no otherconcurrent write grant for the streamlet 4102 at the moment).

When the channel manager 214 allocates a new streamlet (e.g., streamlet4102) for a request for a grant from an MX node (e.g., MX node 204) towrite to a channel (e.g., foo), the channel manager 214 assigns to thestreamlet its TTL, which will expire after TTLs of other streamlets thatare already in the channel's stream. For instance, the channel manager214 can assign to each streamlet of the channel foo's channel stream aTTL of 3 minutes when allocating the streamlet. That is, each streamletwill expire 3 minutes after it is allocated (created) by the channelmanager 214. Since a new streamlet is allocated after a previousstreamlet is closed (e.g., filled entirely or expired), in this way, thechannel foo's channel stream comprises streamlets that each expiressequentially after its previous streamlet expires. For instance, asshown in an example channel stream 430 of the channel foo in FIG. 4A,streamlet 4098 and streamlets before 4098 have expired (as indicated bythe dotted-lined gray-out boxes). Messages stored in these expiredstreamlets are not available for reading for subscribers of the channelfoo. Streamlets 4099, 4100, 4101, and 4102 are still active (notexpired). The streamlets 4099, 4100, and 4101 are closed for writing,but still are available for reading. The streamlet 4102 is available forreading and writing, at the moment when the message M28 was stored inthe streamlet 4102. At a later time, the streamlet 4099 will expire,following by the streamlets 4100, 4101, and so on.

FIG. 4B is a data flow diagram of an example method for subscribing to achannel of a messaging system. In FIG. 4B, a subscriber 480 establishesa connection 462 with an MX node 461 of the messaging system 100.Subscriber 482 establishes a connection 463 with the MX node 461.Subscriber 485 establishes a connection 467 with an MX node 468 of themessaging system 100. Here, the MX nodes 461 and 468 can respectivelycommunicate (464) with the channel manager 214 and one or more Q nodesin the messaging system 100 via the internal network 218.

A subscriber (e.g., subscriber 480) can subscribe to the channel foo ofthe messaging system 100 by establishing a connection (e.g., 462) andsending a request for subscribing to messages of the channel foo to anMX node (e.g., MX node 461). The request (e.g., in JSON key/value pairs)can include a channel name, such as, for example, “foo.” When receivingthe subscribe request, the MX node 461 can send to the channel manager214 a request for a read grant for a streamlet in the channel foo'schannel stream.

By way of illustration, assume that at the current moment the channelfoo's channel stream 431 includes active streamlets 4102, 4103, and4104, as shown in FIG. 4B. The streamlets 4102 and 4103 each are full.The streamlet 4104 stores messages of the channel foo, including thelast message (at the current moment) stored at a position 47731.Streamlets 4101 and streamlets before 4101 are invalid, as theirrespective TTLs have expired. Note that the messages M78, M11, M13, M79,M14, M12, and M26 stored in the streamlet 4101, described earlier inreference to FIG. 4A, are no longer available for subscribers of thechannel foo, since the streamlet 4101 is no longer valid, as its TTL hasexpired. As described earlier, each streamlet in the channel foo'schannel stream has a TTL of 3 minutes, thus only messages (as stored instreamlets of the channel foo) that are published to the channel foo(i.e., stored into the channel's streamlets) no earlier than 3 minutesfrom the current time can be available for subscribers of the channelfoo.

The MX node 461 can request a read grant for all available messages inthe channel foo, for example, when the subscriber 480 is a newsubscriber to the channel foo. Based on the request, the channel manager214 provides the MX node 461 a read grant to the streamlet 4102 (on theQ node 208) that is the earliest streamlet in the active streamlets ofthe channel foo (i.e., the first in the sequence of the activestreamlets). The MX node 461 can retrieve messages in the streamlet 4102from the Q node 208, using the method for reading data from a streamletdescribed earlier in reference to FIG. 3B, for example. Note that themessages retrieved from the streamlet 4102 maintain the same order asstored in the streamlet 4102. However, other arrangements or ordering ofthe messages in the streamlet are possible. In various implementations,when providing messages stored in the streamlet 4102 to the MX node 461,the Q node 208 can buffer (e.g., in a local data buffer) the messagesand send the messages to the MX node 461 when the buffer messages reacha predetermined number or size (e.g., 200 messages) or a predeterminedtime (e.g., 50 milliseconds) has elapsed. For instance, the Q node 208can send the channel foo's messages (from the streamlet 4102) to the MXnode 461 200 messages at a time or in every 50 milliseconds. Otheracknowledgement scheduling algorithms, such as Nagle's algorithm, can beused.

After receiving the last message in the streamlet 4102, the MX node 461can send an acknowledgement to the Q node 208, and send to the channelmanager 214 another request (e.g., for a read grant) for the nextstreamlet in the channel stream of the channel foo. Based on therequest, the channel manager 214 provides the MX node 461 a read grantto the streamlet 4103 (on Q node 472) that logically follows thestreamlet 4102 in the sequence of active streamlets of the channel foo.The MX node 461 can retrieve messages stored in the streamlet 4103,e.g., using the method for reading data from a streamlet describedearlier in reference to FIG. 3B, until it retrieves the last messagestored in the streamlet 4103. The MX node 461 can send to the channelmanager 214 yet another request for a read grant for messages in thenext streamlet 4104 (on Q node 474). After receiving the read grant, theMX node 461 retrieves message of the channel foo stored in the streamlet4104, until the last message at the position 47731. Similarly, the MXnode 468 can retrieve messages from the streamlets 4102, 4103, and 4104(as shown with dotted arrows in FIG. 4B), and provide the messages tothe subscriber 485.

The MX node 461 can send the retrieved messages of the channel foo tothe subscriber 480 (via the connection 462) while receiving the messagesfrom the Q node 208, 472, or 474. In various implementations, the MXnode 461 can store the retrieved messages in a local buffer. In thisway, the retrieved messages can be provided to another subscriber (e.g.,subscriber 482) when the other subscriber subscribes to the channel fooand requests the channel's messages. The MX node 461 can remove messagesstored in the local buffer that each has a time of publication that hasexceeded a predetermined time period. For instance, the MX node 461 canremove messages (stored in the local buffer) with respective times ofpublication exceeding 3 minutes. In some implementations, thepredetermined time period for keeping messages in the local buffer on MXnode 461 can be the same as or similar to the time-to-live duration of astreamlet in the channel foo's channel stream, since at a given moment,messages retrieved from the channel's stream do not include those instreamlets having respective time-to-lives that had already expired.

The messages retrieved from the channel stream 431 and sent to thesubscriber 480 (by the MX node 461) are arranged in the same order asthe messages were stored in the channel stream, although otherarrangements or ordering of the messages are possible. For instance,messages published to the channel foo are serialized and stored in thestreamlet 4102 in a particular order (e.g., M27, M31, M29, M30, and soon), then stored subsequently in the streamlet 4103 and the streamlet4104. The MX node retrieves messages from the channel stream 431 andprovides the retrieved messages to the subscriber 480 in the same orderas the messages are stored in the channel stream: M27, M31, M29, M30,and so on, followed by ordered messages in the streamlet 4103, andfollowed by ordered messages in the streamlet 4104.

Instead of retrieving all available messages in the channel stream 431,the MX node 461 can request a read grant for messages stored in thechannel stream 431 starting from a message at particular position, e.g.,position 47202. For instance, the position 47202 can correspond to anearlier time instance (e.g., 10 seconds before the current time) whenthe subscriber 480 was last subscribing to the channel foo (e.g., via aconnection to the MX node 461 or another MX node of the messaging system100). The MX node 461 can send to the channel manager 214 a request fora read grant for messages starting at the position 47202. Based on therequest, the channel manager 214 provides the MX node 461 a read grantto the streamlet 4104 (on the Q node 474) and a position on thestreamlet 4104 that corresponds to the channel stream position 47202.The MX node 461 can retrieve messages in the streamlet 4104 startingfrom the provided position, and send the retrieved messages to thesubscriber 480.

As described above in reference to FIGS. 4A and 4B, messages publishedto the channel foo are serialized and stored in the channel's streamletsin a particular order. The channel manager 214 maintains the orderedsequence of streamlets as they are created throughout their respectivetime-to-lives. Messages retrieved from the streamlets by an MX node(e.g., MX node 461, or MX node 468) and provided to a subscriber can be,in some implementations, in the same order as the messages are stored inthe ordered sequence of streamlets. In this way, messages sent todifferent subscribers (e.g., subscriber 480, subscriber 482, orsubscriber 485) can be in the same order (as the messages are stored inthe streamlets), regardless which MX nodes the subscribers are connectedto.

In various implementations, a streamlet stores messages in a set ofblocks of messages. Each block stores a number of messages. Forinstance, a block can store two hundred kilobytes of messages. Eachblock has its own time-to-live, which can be shorter than thetime-to-live of the streamlet holding the block. Once a block's TTL hasexpired, the block can be discarded from the streamlet holding theblock, as described in more detail below in reference to FIG. 4C.

FIG. 4C is an example data structure for storing messages of a channelof a messaging system. As described with the channel foo in reference toFIGS. 4A and 4B, assume that at the current moment the channel foo'schannel stream 432 includes active streamlets 4104 and 4105, as shown inFIG. 4C. Streamlet 4103 and streamlets before 4103 are invalid, as theirrespective TTLs have expired. The streamlet 4104 is already full for itscapacity (e.g., as determined by a corresponding write grant) and isclosed for additional message writes. The streamlet 4104 is stillavailable for message reads. The streamlet 4105 is open and is availablefor message writes and reads.

By way of illustration, the streamlet 4104 (e.g., a computing processrunning on the Q node 474 shown in FIG. 4B) currently holds two blocksof messages. Block 494 holds messages from channel positions 47301 to47850. Block 495 holds messages from channel positions 47851 to 48000.The streamlet 4105 (e.g., a computing process running on another Q nodein the messaging system 100) currently holds two blocks of messages.Block 496 holds messages from channel positions 48001 to 48200. Block497 holds messages starting from channel position 48201, and stillaccepts additional messages of the channel foo.

When the streamlet 4104 was created (e.g., by a write grant), a firstblock (sub-buffer) 492 was created to store messages, e.g., from channelpositions 47010 to 47100. Later on, after the block 492 had reached itscapacity, another block 493 was created to store messages, e.g., fromchannel positions 47111 to 47300. Blocks 494 and 495 were subsequentlycreated to store additional messages. Afterwards, the streamlet 4104 wasclosed for additional message writes, and the streamlet 4105 was createdwith additional blocks for storing additional messages of the channelfoo.

In this example, the respective TTL's of blocks 492 and 493 had expired.The messages stored in these two blocks (from channel positions 47010 to47300) are no longer available for reading by subscribers of the channelfoo. The streamlet 4104 can discard these two expired blocks, e.g., byde-allocating the memory space for the blocks 492 and 493. The blocks494 or 495 could become expired and be discarded by the streamlet 4104,before the streamlet 4104 itself becomes invalid. Alternatively,streamlet 4104 itself could become invalid before the blocks 494 or 495become expired. In this way, a streamlet can hold one or more blocks ofmessages, or contain no block of messages, depending on respective TTLsof the streamlet and blocks, for example.

A streamlet, or a computing process running on a Q node in the messagingsystem 100, can create a block for storing messages of a channel byallocating a certain size of memory space from the Q node. The streamletcan receive, from an MX node in the messaging system 100, one message ata time and store the received message in the block. Alternatively, theMX node can assemble (i.e., buffer) a group of messages and send thegroup of messages to the Q node. The streamlet can allocate a block ofmemory space (from the Q node) and store the group of messages in theblock. The MX node can also perform compression on the group ofmessages, e.g., by removing a common header from each message orperforming other suitable compression techniques.

Referring again to FIG. 2, in some examples, the systems and methodsdescribed herein balance load among the Q nodes for one or morechannels. For example, when selecting a Q node to host a streamlet for achannel, the channel manager 214 can select the Q node based on itspresent workload (also referred to herein as “load”) and/or based on anexpected workload the Q node will have once the hosting begins. Theworkload of the Q node and other nodes can be determined using load datathat provides an indication of how active or busy the Q nodes are at thecurrent time and/or are projected to be in the future. The load data fora given Q node can include one or more load metrics, such as, forexample, information about the number of messages being handled orprocessed by the Q node. In various implementations, the load data is orcan include a combination of two or more load metrics. For example, thecombination can be linear, non-linear, weighted, or un-weighted,although other combinations are possible. In one example, the load datacan be a weighted combination of load metrics, with weights for thecombination determined through experimental measurements and analysis ofsystem performance. Linear regression or other data-fitting techniquescan be used to determine the weights and/or the load metrics that havethe greatest influence on workload and system performance. In someinstances, the load data can include node-specific data representingloads on one or more Q nodes and/or channel-specific data representingloads associated with one or more channels. The load data can be orinclude, for example, a combination (e.g., a weighted combination) ofnode-specific data and/or channel-specific data.

In certain examples, the node-specific load data can include the rate atwhich messages are being written to the Q node and/or the rate at whichmessages are being delivered from the Q node. In general, the higher therate at which the Q node is sending and/or receiving messages, thehigher the workload is for the Q node. The rate of transfer to or fromthe Q node can be compared with maximum or threshold transfer rates forthe Q node. The threshold transfer rates can be determined statically,for example, by observing a system configuration, such as a networkinterface (e.g., Ethernet) device's capacity. Alternatively oradditionally, the threshold transfer rates can be determineddynamically, for example, by observing the maximum transfer rates atwhich response latencies remain at a pre-defined level. An initialthreshold transfer rate can also be determined experimentally, such asduring benchmarking of the system. In various instances, if the rate oftransfer to and/or from the Q node is at, near or exceeds the maximumavailable or threshold transfer rate, the workload and/or the possiblefuture workload for the Q node can be considered high, such that the Qnode is less likely to be selected for hosting of additional streamletsat that time. In some examples, message transfer rates are measured interms of the number of messages per time (e.g., messages per second)and/or the data transfer rate (e.g., bytes per second).

In some instances, the node-specific load data can include the number ofstreamlets or messages currently stored by the Q nodes. The storage inthe Q node can be compared with a maximum or threshold storage value forthe Q node. The threshold storage value can be determined statically,for example, by observing the system configuration and dedicating aportion of the system memory (e.g., RAM), such as 70%, 80%, 90% or othersuitable percentage, to the application. An initial threshold storagevalue can be determined experimentally, such as during benchmarking ofthe system. In general, when storage in a Q node is at, near or exceedsthe threshold storage value, the workload and/or possible futureworkload can be considered high and/or the Q node may have limited spacefor additional storage. In such cases, the Q node is less likely to beselected at that time for storage of additional streamlets. The numberof messages stored by a Q node may be measured, for example, in bytes orin number of messages.

Alternatively or additionally, node-specific load data for a given Qnode can include information about the number of channels or channelportions (i.e., streamlets) currently being hosted or processed by the Qnode. The number of channels or channel portions being hosted orprocessed by the Q node can be compared with a maximum or thresholdnumber of channels. If the number of channels hosted by the Q node isat, near or exceeds the maximum number of channels, the workload of theQ node and/or the possible future workload can be considered high. Invarious instances, the maximum number of channels can be limited byand/or determined from the system memory and/or the CPU and networkoverhead of keeping or maintaining a channel. An initial maximum numberof channels can be determined experimentally, such as duringbenchmarking of the system. As messaging activity for a channelincreases, the workload for the Q node hosting the channel can beexpected to increase. The channel manager 214 can monitor trends inchannel messaging activity to predict how the hosting of streamlets willinfluence Q node loads. If the expected workload associated with hostinga streamlet will be too high for a Q node, the channel manager 214 canselect a different Q node that has sufficient workload capacity and/oravailable storage to host the streamlet.

In some instances, the node-specific load data for a Q node is measuredbased on the number of MX nodes that have been given read and/or writeaccess to the Q node. The workload of a Q node can increase as thenumber of MX nodes having read/write access to the Q node increases.Additionally, the number of MX nodes having read/write access to the Qnode can provide an indication of the possible future workload for the Qnode. For example, when a large number of MX nodes have read/writeaccess to the Q node, the potential for high message transfer ratesexists, even though current message transfer rates may not be high. Insuch cases, the MX nodes can put higher demands on the Q node, as theactivity level on corresponding channels increases. The node-specificload data for a Q node can include, in some instances, informationregarding (i) the number of received requests from MX nodes to accessstreamlets stored on the Q node and/or (ii) the number of permissionsgranted to the MX nodes to access the streamlets.

In general, the node-specific load data for a Q node can include theprocessing rate for the Q node. A computation or processing rate for theQ node can be calculated and compared with a threshold or maximumprocessing value for the Q node. The threshold processing (CPU) valuecan be, for example, between 30% and 70% of a Q node CPU limit, toaccount for spikes, although other threshold processing values arepossible. In one example, the threshold processing value can bedetermined by observing system behavior under actual production load anddetermining safe constraints, for example, by determining a level atwhich the system becomes unstable, which may be indicated byoscillations in traffic or processing rates. In one example, thethreshold processing value can be equal to one-half or one-third of theprocessing rate corresponding to the onset of system instability,although other threshold processing values are possible. When theprocessing rate for the Q node is at, near, or exceeds the thresholdprocessing value, the workload for the Q node can be considered high.With a high workload, the Q node is less likely to be selected by thechannel manager 214 to host a new streamlet.

In addition to monitoring the Q node workloads, the channel manager 214can also monitor rates of change in the workloads. The node-specificload data can include, for example, an indication of how the messagetransfer rates, message storage amounts, number of channels hosted,number of MX node connections, the processing rate for the Q node,and/or other load metrics are changing over time. The rates of changecan be or include, for example, a derivative or slope associated withthe load metrics. The rates of change can be used to predict what theworkload will be in the future for the Q node. For example, the channelmanager 214 can use the current workload and the rate of change toextrapolate (e.g., linearly) from the current workload to a predictedfuture workload.

In general, to determine if a current or future workload of a Q node ishigh, the systems and methods (e.g., the channel manager 214) cancompare the current or future workload (e.g., a message transfer rate ora storage rate) with a threshold value. The threshold value can be, forexample, a maximum value that should not be exceeded, to avoidperformance issues. In some instances, the threshold value can bedetermined through experimental observation and/or is chosen to be aworkload above which system performance is reduced or otherwise notoptimal. The workload for a Q node can be expressed as a raw load levelor as a percentage of the threshold value. In general, when the currentor predicted workload is at or near (or even exceeds) the thresholdvalue, the Q node can be considered overloaded and is less likely to beselected to host a new streamlet.

Additionally or alternatively, the systems and methods (e.g., thechannel manager 214) can balance loads on the Q nodes by consideringchannel-specific data. In general, channel-specific data relates toinformation about a channel for which a new streamlet will be hosted.Channel-specific data can include, for example, the number ofsubscribers to a channel and/or the number of publishers to the channel.If the number of subscribers and/or publishers to the channel is high,an anticipated load associated with a new streamlet for the channel canalso be high. Likewise, the channel-specific data can include a rate atwhich messages are published to the channel. A high publication rate fora channel is generally an indication that a workload associated with anew streamlet for the channel will be high. In some examples, thechannel-specific data includes the number of interface nodes havingpermission to access the channel. In general, when a large number ofinterface nodes can access a channel, the expected workload for thechannel will be high, for example, due to more requests to read from orwrite to the channel. Accordingly, channel-specific data can allow thechannel manager 214 to predict a workload associated with a particularchannel. The channel manager 214 can use the predicted workload todetermine how much work will be associated with hosting a new streamletfor the channel. The channel manager 214 can then use the predictedworkload to choose an appropriate Q node for hosting the new streamlet.For example, if the predicted workload for the streamlet is expected tobe high, based on the channel-specific data, the channel manager 214 canchoose a Q node having a workload that is low enough to handle the highworkload associated with the new streamlet.

In various examples, the node-specific data and/or the channel-specificdata can consider or include geographic location. For example, ifchannel activity is primarily expected to be in a particular geographiclocation (e.g., New Zealand), then the channel manager 214 can select aQ node that resides in or near the geographic location (e.g., in a NewZealand data center).

In some examples, the one or more Q nodes and/or the one or more MXnodes provide the channel manager 214 with the load data, including thenode-specific data and/or the channel-specific data. The Q nodes can beconfigured, for example, to monitor their message transfer rates,messages storage amounts, MX node connections, etc., and any associatedrates of change, and provide that information (e.g., node-specific data)to the channel manager 214. The same or similar node-specificinformation can be collected by MX nodes and/or provided to the channelmanager 214 by MX nodes. The channel-specific data can likewise becollected by Q nodes and/or MX nodes and sent to the channel manager214. For example, the Q nodes and/or the MX nodes can monitor one ormore channels to determine the number of subscribers, the number ofpublishers, the rate of message publication, and/or the number of MXconnections for the channels.

In general, the channel manager 214 uses the load data (i.e.,node-specific data and/or channel-specific data) to balance loads amongthe various Q nodes. For example, the channel manager 214 can use theload data to select the next Q node for hosting a new streamlet. Thenext Q node can be chosen based on its current workload or projectedfuture workload, compared to other Q nodes in the system. For example,when the channel manager 214 is selecting a Q node to host a streamletfor a channel, the channel manager 214 can choose a Q node that has thelowest workload or the lowest projected future workload among theavailable Q nodes. To predict the future workload, the channel manager214 can estimate an additional workload associated with a future hostingtask and add the additional workload to the current workload for the Qnode. The channel manager 214 can also consider how many streamletsbeing hosted by the Q node will expire in the future, thereby reducingthe Q node's workload.

In some cases, the channel manager 214 can predict a Q node's workloadat a future time as follows: future workload=current workload+expectedchange in workload. The current workload is generally a Q node'sworkload at a current time. The expected change in workload is anexpected difference between the current workload and the expectedworkload at the future time. The expected change in workload can bedetermined based on, for example, the predicted increase in workload(e.g., due to hosting new streamlets and/or increases in channelactivity) and the predicted decrease in workload (e.g., due to streamletexpiration and/or decreases in channel activity).

In certain instances, the systems and methods (e.g., the channel manager214 and/or the Q nodes) can monitor the workloads of the various Q nodesto determine when new streamlets need to be opened or closed. Forexample, the channel manager 214 can decide to close a streamlet on a Qnode when a workload of the Q node is getting high. The channel manager214 can then open a new streamlet for the corresponding channel on adifferent Q node, preferably selected based on the load data and Q nodeworkloads, as described herein.

In various instances, when a first streamlet will be closed and a secondstreamlet immediately following the first streamlet will be opened, thechannel manager 214 can open the second streamlet on the Q node that ishosting the first streamlet. When deciding to use the same Q node forthe first and second streamlets, the channel manager 214 can firstconfirm that the workload of the Q node is below a threshold level, suchthat opening the second streamlet will not overload the Q node.Alternatively, if the workload of the Q node is above the thresholdlevel, the channel manager 214 can select a different Q node, having aworkload below the threshold level, to host the second streamlet.

In general, when selecting Q nodes to host new streamlets, the systemsand methods can attempt to balance workloads among the available Qnodes. When a new streamlet will be opened, for example, the channelmanager 214 can determine an expected workload associated with hostingthe new streamlet. The channel manager 214 can then select a Q node tohost the new streamlet based on the expected workload associated withhosting the streamlet. The Q node can be selected such that workloadsare distributed equally across the Q nodes of the system. To determineworkload inequality among the Q nodes, the channel manager 214 candetermine a standard deviation of the workload distribution and select Qnodes for new hosting tasks in an effort to minimize the standarddeviation. Other measurements of workload inequality can include, forexample, the difference between a maximum workload and a minimumworkload among the Q nodes, or the variance among the Q nodes. Ingeneral, when selecting the next Q node for hosting a streamlet, thechannel manager 214 can make a Q node selection that reduces workloadinequality among the Q nodes.

In certain examples, the channel manager 214 can send a request to a Qnode to terminate hosting of a streamlet. The request to terminate canbe sent, for example, when a determination is made that there are nosubscribes to the channel associated with the streamlet and/or when atime-to-live for the streamlet has expired. In response to the request,the Q node can terminate the hosting of the streamlet and inform thechannel manager 214 that the hosting has been terminated. Terminatingthe hosting of the streamlet can include, for example, closing thestreamlet to further publication, closing the streamlet to furtherreading, and/or deleting message data associated with the streamlet. Adecision to close a streamlet can be based on, for example, thedetermination that the size of the streamlet exceeds a threshold size,the determination that the age of the streamlet exceeds a threshold age(e.g., a TTL), and/or the determination that the hosting node hasexperienced a communication failure.

In some instances, an MX node informs the channel manager 214 about arequest from a publisher to publish to a new channel. In such a case,the channel manager 214 can determine that the channel does not existand, in response, can select a Q node to host a streamlet for the newchannel. The Q node selection can be performed using the techniquesdescribed herein. The channel manager 214 can select the same Q node ora different Q node to host additional streamlets for the new channel(e.g., when a preceding or youngest streamlet is closed to furtherpublication).

FIG. 5 is a flowchart of an example method for balancing workload amongQ nodes of a publish-subscribe system. The method can be implementedusing a channel manager, such as, for example, the channel manager 214of the messaging system 100. The method begins by selecting (step 502),from a plurality of hosting nodes (i.e., Q nodes) of a publish-subscribesystem, a first hosting node (i.e., a first Q node) to temporarily hosta portion of a channel of the publish-subscribe system. In certaininstances, temporarily hosting the channel portion includes temporarilystoring one or more messages published to the channel, and temporarilyproviding, to a plurality of subscribers to the channel, access to theone or more messages. The method also includes sending (step 504), tothe first hosting node of the publish-subscribe system, a request totemporarily host the channel portion. A request to access the channelportion is received (step 506) from an interface node (i.e., an MX node)of the publish-subscribe system. Permission to access the channelportion is granted (step 508) to the interface node. In general,selecting the first hosting node to temporarily host the channel portionincludes selecting the first hosting node from the plurality of hostingnodes based, at least in part, on load data that includes node-specificdata representing loads on the plurality of hosting nodes and/orchannel-specific data representing a load associated with the channel.

Embodiments of the subject matter and the operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Embodiments of the subject matterdescribed in this specification can be implemented as one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on computer storage medium for execution by, or tocontrol the operation of, data processing apparatus. Alternatively or inaddition, the program instructions can be encoded on anartificially-generated propagated signal, e.g., a machine-generatedelectrical, optical, or electromagnetic signal, that is generated toencode information for transmission to suitable receiver apparatus forexecution by a data processing apparatus. A computer storage medium canbe, or be included in, a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memoryarray or device, or a combination of one or more of them. Moreover,while a computer storage medium is not a propagated signal, a computerstorage medium can be a source or destination of computer programinstructions encoded in an artificially-generated propagated signal. Thecomputer storage medium can also be, or be included in, one or moreseparate physical components or media (e.g., multiple CDs, disks, orother storage devices).

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing. The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram may, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languageresource), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub-programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. The essential elements of a computer area processor for performing actions in accordance with instructions andone or more memory devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom or transfer data to, or both, one or more mass storage devices forstoring data, e.g., magnetic disks, magneto-optical disks, opticaldisks, or solid state drives. However, a computer need not have suchdevices. Moreover, a computer can be embedded in another device, e.g., amobile telephone, a personal digital assistant (PDA), a mobile audio orvideo player, a game console, a Global Positioning System (GPS)receiver, or a portable storage device (e.g., a universal serial bus(USB) flash drive), to name just a few. Devices suitable for storingcomputer program instructions and data include all forms of non-volatilememory, media and memory devices, including, by way of example,semiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse, a trackball, a touchpad,or a stylus, by which the user can provide input to the computer. Otherkinds of devices can be used to provide for interaction with a user aswell; for example, feedback provided to the user can be any form ofsensory feedback, e.g., visual feedback, auditory feedback, or tactilefeedback; and input from the user can be received in any form, includingacoustic, speech, or tactile input. In addition, a computer can interactwith a user by sending resources to and receiving resources from adevice that is used by the user; for example, by sending web pages to aweb browser on a user's client device in response to requests receivedfrom the web browser.

Embodiments of the subject matter described in this specification can beimplemented in a computing system that includes a back-end component,e.g., as a data server, or that includes a middleware component, e.g.,an application server, or that includes a front-end component, e.g., aclient computer having a graphical user interface or a Web browserthrough which a user can interact with an implementation of the subjectmatter described in this specification, or any combination of one ormore such back-end, middleware, or front-end components. The componentsof the system can be interconnected by any form or medium of digitaldata communication, e.g., a communication network. Examples ofcommunication networks include a local area network (“LAN”) and a widearea network (“WAN”), an inter-network (e.g., the Internet), andpeer-to-peer networks (e.g., ad hoc peer-to-peer networks).

The computing system can include clients and servers. A client andserver are generally remote from each other and typically interactthrough a communication network. The relationship of client and serverarises by virtue of computer programs running on the respectivecomputers and having a client-server relationship to each other. In someembodiments, a server transmits data (e.g., an HTML page) to a clientdevice (e.g., for purposes of displaying data to and receiving userinput from a user interacting with the client device). Data generated atthe client device (e.g., a result of the user interaction) can bereceived from the client device at the server.

A system of one or more computers can be configured to performparticular operations or actions by virtue of having software, firmware,hardware, or a combination of them installed on the system that inoperation causes or cause the system to perform the actions. One or morecomputer programs can be configured to perform particular operations oractions by virtue of including instructions that, when executed by dataprocessing apparatus, cause the apparatus to perform the actions.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results. In addition, the processesdepicted in the accompanying figures do not necessarily require theparticular order shown, or sequential order, to achieve desirableresults. In certain implementations, multitasking and parallelprocessing may be advantageous.

What is claimed is:
 1. A load-balancing method for a publish-subscribesystem, the load-balancing method comprising: selecting, by one or morecomputer processors, a first hosting node from a plurality of hostingnodes based, at least in part, on load data comprising node-specificdata representing loads on the plurality of hosting nodes andchannel-specific data representing a load associated with a channelcomprising a channel portion to be temporarily offloaded; sending, bythe one or more computer processors, a request to the first hosting nodeto temporarily host the channel portion of the channel, wherein therequest to the first hosting node to temporarily host the channelportion comprises an indication for one or more messages published tothe channel to be temporarily stored and for access to the one or moremessages to be temporarily provided to a plurality of subscribers, andwherein the one or more messages were previously stored on a secondhosting node; receiving a request to access the channel portion; andgranting permission to access the channel portion.
 2. The load-balancingmethod of claim 1, wherein the node-specific data comprises one or moreload metrics selected from at least one of: a number of channel portionsbeing temporarily hosted by the respective hosting nodes, a number ofinterface nodes having permission to access the respective hostingnodes, a data reception rate of the respective hosting nodes, a datatransmission rate of the respective hosting nodes, a storage utilizationof the respective hosting nodes, or a processing rate of the respectivehosting nodes.
 3. The load-balancing method of claim 1, furthercomprising: receiving at least a portion of the node-specific data fromthe plurality of hosting nodes.
 4. The load-balancing method of claim 1,further comprising: determining at least a portion of the node-specificdata based, at least in part, on received requests to access the channelportion and on permissions granted to access the channel portion.
 5. Theload-balancing method of claim 1, wherein the channel-specific datacomprises one or more load metrics selected from at least one of: anumber of subscribers to the channel, a number of publishers to thechannel, a rate at which messages are published to the channel, a rateat which messages are read from the channel, a number of interface nodeshaving permission to access the channel, or a channel portion size forthe channel.
 6. The load-balancing method of claim 1, furthercomprising: receiving at least a portion of the channel-specific datafrom at least one of a hosting node or an interface node.
 7. Theload-balancing method of claim 1, wherein selecting the first hostingnode from the plurality of hosting nodes based, at least in part, on theload data comprises: determining, based at least in part on thenode-specific data, that a load on the first hosting node is lowestamong respective loads on the hosting nodes; and selecting the firsthosting node based, at least in part, on the determination.
 8. Theload-balancing method of claim 1, wherein selecting the first hostingnode from the plurality of hosting nodes based, at least in part, on theload data comprises: determining, based at least in part on thenode-specific data, that a load on the first hosting node is below athreshold load level; and selecting the first hosting node based, atleast in part, on the determination.
 9. The load-balancing method ofclaim 1, wherein selecting the first hosting node from the plurality ofhosting nodes based, at least in part, on the load data comprises:determining, based at least in part on a portion of the node-specificdata corresponding to the first hosting node and on a portion of thechannel-specific data corresponding to the channel, an expected load onthe first hosting node that would result from the first hosting nodehosting the portion of the channel; determining that the expected loadon the first hosting node is below a threshold load level; and selectingthe first hosting node based, at least in part, on the determinationthat the expected load on the first hosting node is below the thresholdload level.
 10. The load-balancing method of claim 1, wherein thechannel portion comprises a first portion of the channel, wherein thechannel further comprises a second channel portion, and whereinselecting the first hosting node from the plurality of hosting nodesbased, at least in part, on the load data comprises: determining thatthe first hosting node hosts the second channel portion; determiningthat a load on the first hosting node is below a threshold load level;and selecting the first hosting node based, at least in part, on thedeterminations that the first hosting node hosts the second channelportion and that the load on the first hosting node is below thethreshold load level.
 11. The load-balancing method of claim 1, whereinthe Channel portion comprises a first portion of the channel, whereinthe channel further comprises a second channel portion, and whereinselecting the first hosting node from the plurality of hosting nodesbased, at least in part, on the load data comprises: determining thatthe second hosting node hosts the second channel portion; determiningthat a load on the second hosting node is above a threshold load level;determining that a load on the first hosting node is below the thresholdload level; and selecting the first hosting node based, at least inpart, on the determinations that the load on the second hosting node isabove the threshold load level and that the load on the first hostingnode is below the threshold load level.
 12. The load-balancing method ofclaim 1, wherein selecting the first hosting node from the plurality ofhosting nodes based, at least in part, on the load data comprises:determining, based at least in part on a portion of the channel-specificdata, an expected load associated with hosting the channel portion;determining, based at least in part on the node-specific data and on theexpected load associated with hosting the channel portion, that hostingthe channel portion on the first hosting node would reduce inequality ofload distribution among the hosting nodes; and selecting the firsthosting node based, at least in part, on the determination that hostingthe channel portion on the first hosting node would reduce inequality ofload distribution among the hosting nodes.
 13. A computing device,comprising: a channel manager node operable to: select a first hostingnode from a plurality of hosting nodes based, at least in part, on loaddata comprising node-specific data representing loads on the pluralityof hosting nodes and channel-specific data representing a loadassociated with a channel comprising a channel portion to be temporarilyoffloaded; send a request to the first hosting node to temporarily hostthe channel portion of the channel, wherein the request to the firsthosting node to temporarily host the channel portion comprises anindication for one or more messages published to the channel to betemporarily stored and for access to the one or more messages to betemporarily provided to a plurality of subscribers to the channel, andwherein the one or more messages were previously stored on a secondhosting node; receive, from an interface node, a request to access thechannel portion; and grant, to the interface node, permission to accessthe channel portion.
 14. The computing device of claim 13, wherein toselect the first hosting node from the plurality of hosting nodes based,at least in part, on the load data the channel manager node is furtherto: determine, based at least in part on the node-specific data, that aload on the first hosting node is lowest among respective loads on thehosting nodes; and select the first hosting node based, at least inpart, on the determination.
 15. The computing device of claim 13,wherein to select the first hosting node from the plurality of hostingnodes based, at least in part, on the load data the channel manager nodeis further to: determine, based at least in part on the node-specificdata, that a load on the first sting node is below a threshold loadlevel; and select the first hosting node based, at least in part, on thedetermination.
 16. The computing device of claim 13, wherein to selectthe first hosting node from the plurality of hosting nodes based, atleast in part, on the load data the channel manager node is further to:determine, based at least in part on a portion of the node-specific datacorresponding to the first hosting node and on a portion of thechannel-specific data corresponding to the channel, an expected load onthe first hosting node that would result from the first hosting nodehosting the portion of the channel; determine that the expected load onthe first hosting node is below a threshold load level; and select thefirst hosting node based, at least in part, on the determination thatthe expected load on the first hosting node is below the threshold loadlevel.
 17. The computing device of claim 13, wherein the channel portioncomprises a first portion of the channel, wherein the channel furthercomprises a second channel portion, and wherein to select the firsthosting node from the plurality of hosting nodes based, at least inpart, on the load data the channel manager node is further to: determinethat the first hosting node hosts the second channel portion; determinethat a load on the first hosting node is below a threshold load level;and select the first hosting node based, at least in part, on thedeterminations that the first hosting node hosts the second channelportion and that the load on the first hosting node is below thethreshold load level.
 18. The computing device of claim 13, wherein thechannel portion comprises a first portion of the channel, wherein thechannel further comprises a second channel portion, and wherein toselect the first hosting node from the plurality of hosting nodes based,at least in part, on the load data the channel manager node is furtherto: determine that the second hosting node hosts the second channelportion; determine that a load on the second hosting node is above athreshold load level; determine that a load on the first hosting node isbelow the threshold load level; and select the first hosting node based,at least in part, on the determinations that the load on the secondhosting node is above the threshold load level and that the load on thefirst hosting node is below the threshold load level.
 19. The computingdevice of claim 13, wherein to select the first hosting node from theplurality of hosting nodes based, at least in part, on the load data thechannel manager node is further to: determine, based at least in part ona portion of the channel-specific data, an expected load associated withhosting the channel portion; determine, based at least in part on thenode-specific data and on the expected load associated with hosting thechannel portion, that hosting the channel portion on the first hostingnode would reduce inequality of load distribution among the hostingnodes; and select the first hosting node based, at least in part, on thedetermination that hosting the channel portion on the first hosting nodewould reduce inequality of load distribution among the hosting nodes.20. A non-transitory machine-readable medium having instructions storedthereon that, when executed by one or more computer processors, causethe one or more computer processors to: select a first hosting node froma plurality of hosting nodes based, at least in part, on load datacomprising node-specific data representing loads on the plurality ofhosting nodes and channel-specific data representing a load associatedwith a channel comprising a channel portion to be temporarily offloaded;send, to the first hosting node, a request to temporarily host thechannel portion of the channel, wherein the request to the first hostingnode to temporarily host the channel portion comprises an indication forone or more messages published to the channel to be temporarily storedand for access to the one or more messages to be temporarily provided toa plurality of subscribers, and wherein the one or more messages werepreviously stored on a second hosting node; receive a request to accessthe channel portion; and grant permission to access the channel portion.